For some reason I had assumed that bloodletting wasn’t very common in the Middle Ages, but my current reading about medieval medicine has set me right. Even in the early Middle Ages it was far from unusual.

Bloodletting is a logical consequence of accepting that illness is caused by an imbalance of the humours. As medical texts from the Greek and Arabic-speaking worlds were translated into Latin from the twelfth century onwards, it became even more important as one of the physician’s many skills. Bloodletting was carried out by both surgeons and physicians, even though it was technically a surgical procedure.

One of the purposes of bloodletting was to allow the physician to make a diagnosis. An instruction book, probably written by Maurus of Salerno in the twelfth century, told the physician what to look for in the blood he collected from his patient. The physician was to…